Iraqi security forces pose from atop a building in the village of al-Buseif, south of Mosul, during an offensive to retake the western side of the city from ISIS, February 22, 2017.Credit: Ahmad al-Rubaye, AFP

Thursday's advance is part of a major assault that started five days ago to drive ISIS militants from the western half of Mosul, Iraq's second-largest city.

Two police officers said the airport buildings are now under fire from the Iraqi police forces. They would not provide more details but said troops from the U.S.-led coalition are with the advancing forces, though they didn't specify the nationalities of the foreign forces.

The officials spoke on condition of anonymity because they are not authorized to talk to reporters.

The operation to retake Iraq's second-largest city was officially launched in October and in January its eastern half was declared "fully liberated."